1. Field of the Invention
This invention relates to an image processing method and apparatus, as well as to a method of manufacturing an article recorded on by ink-jet printing. By way of example, the invention relates to an image forming apparatus for converting a full-color image to palette data of a prescribed number of colors and repeatedly printing the palette image, or to an image forming apparatus referred to as a so-called binary printer.
2. Description of the Related Art
RGB color image data composed of eight bits for each of the colors R, G, B amounts to three megabytes in case of an image of 1000.times.1000 pixels. In order to obtain palette data while minimizing a decline in picture quality, use is made of a technique such as the density preservation method or vector quantization method. For example, "Photoshop" software developed by Adobe Systems incorporates technology considered to be equivalent to the functions mentioned above.
Further, a method of compressing the number of colors using the error diffusion method or an improved technique in which a random-number scheme or dither matrix is applied to this method has been proposed.
A number of problems are encountered in the prior art described above.
Specifically, expressing about 1,670,000 colors using eight-bit RGB (i.e., 24-bit) full-color image data is ordinary. The problem is that although picture quality is good, the amount of data is enormous, there is a decline in processing speed and an increase in memory cost. For this reason, a so-called palettizing technique is used, in which the image data is coded to be compressed to a prescribed number of colors, e.g., 256 colors.
On the other hand, to the extent that a binary color printer such as one using the ink-jet printing technique is capable of expressing gradations in density in pixel units, it cannot control the amount of ink jetted per pixel. For this reason, tones are expressed in simulated fashion by controlling the number of dots in a unit of area using the error diffusion method or dither method. Accordingly, when a palettized image, especially a natural picture such as a photograph, or an image having more colors than the number of palette colors is printed using a binary color printer, a problem which arises is a conspicuous deterioration in picture quality.
Further, software such as the aforementioned "Photoshop" subjects image data to palettization using the error diffusion method. However, since such software cannot give sufficient consideration to the characteristics of the ink used by the printer in printing, a problem which arises is that density cannot be preserved accurately.
Furthermore, in a case where a basic image is repeatedly printed, as in the manner of textile printing used in dyeing fabric, the so-called "sweeping" phenomenon, which is a disadvantage of the error diffusion method, occurs at the boundaries or borders of the repeatedly printed basic images. The result is a fainter image at the boundaries and loss of continuity. Herein "sweeping" is a phenomenon which causes blurring of an image when image data appears as black-color data in the following conditions. That is, the black-color data appears when a value of image data at a low-density portion of the image does not exceed a threshold value for binarization after image processing such as an error diffusion method is started, and errors corresponding to a few number of pixels are stored by the error diffusion method.
Furthermore, in order to compress the number of colors to 256 colors by palettization while minimizing a decline in picture quality, use is made of a technique in which vector quantization is carried out based upon the frequency distribution of each color of the image. However, processing takes too much time and picture quality does not stabilize owing to the accuracy of rounding (approximation) to 256 colors.